Create a new event for modules ('destroy') which is emitted from
`module_free()`. It is used by the module loading logic, to handle
when a module is destroyed without properly loading first.
Modules no longer need to emit the "loaded" event manually
if they can load immediately. In that case, the module loading
code will take care of emitting the event. If they can't,
they must return an async spa result, and emit the "loaded" event
when they see fit.
Fixes#1232
This starts breaking up the giant monolith that is the pulse-server.c
code into more manageable chunks by trying to split the module code into
individual compilation units.
